The extreme ultraviolet wavelength of 13.5 nm (nanometers) has been selected for use in microlithography because good reflective optics are available at this wavelength, and the prospect exists that with this wavelength very high patterning rates will be achieved for integrated circuit features as small as 20 nm. In order to achieve this goal the power from 13.5 nm light sources has to be increased several times beyond current practice, but limiting factors have to be overcome to achieve this.
The use of xenon in a Z-pinch discharge represented the first efficient plasma 13.5 nm source, with a conversion efficiency of 0.5% from stored electrical energy to 13.5 nm radiation in a 2% fractional bandwidth, radiated into a 2π steradian solid angle. However, in order to reach the initial goal of 115 W (watts) of power at an “intermediate focus” of the collimation optic that collects 13.5 nm radiation from the source, up to 700 W of 13.5 nm in-band radiation has to be emitted from the source, representing an electrical power input of 140 kW. The Star Pinch source was developed as a viable method of holding the hot plasma distant from any surface, thereby allowing powers of up to 60 kW to be handled (in principle) before the heat load became a major difficulty. However, this represented the capability to only generate one half of the initial 13.5 nm power requirement (using xenon), and did not offer the prospect of additional power scaling beyond the 115 W initial power requirement, whereas 200 W or more would be needed for future production of smaller feature sizes at higher throughput rates.
Xenon (Xe) was originally chosen because it radiated 13.5 nm light more efficiently than other gases, such as oxygen, while at the same time being a non-reactive noble gas that did not interact with the surfaces of the collection optic. However, the principal emission wavelength of xenon is not ideally placed, being at 11 nm rather than 13.5 nm, putting it outside of the range of high reflectivity optics. Other substances, such as tin (Sn8+) and lithium (Li2+) have their principal emissions exactly at 13.5 nm, and hence are more efficient lithography sources than Xe, but each of these is a low vapor pressure metal. The change to metals such as Sn or Li from Xe brought two major challenges: to ensure sufficient vapor density of the metal for a pinch discharge; and to prevent metallic condensation on the collection optic which would degrade its reflectivity. The first major progress toward solution of these objectives was made in relation to the formation of energetic Li+ states via buffered heat pipe containment of metal and excitation via a pulsed hollow cathode discharge. The rewards from the change from xenon to metals in terms of 13.5 nm production efficiency were high: 2% efficiency in Sn discharges and 2% in Li discharges, with a probable efficiency increase to well above 2% once the discharge conditions in Li have been optimized. When these factor-of-four efficiency increases are considered, the 60 kW power limitation of the star pinch discharge is sufficiently high to allow production of 200 W of usable 13.5 nm radiation. The present invention introduces a way to achieve sufficient metal vapor density at the same time as preventing the escape of metal vapor through the wide angle subtended at the source by the collector optic (typically at least 2 steradians).
The use of a heat pipe with a buffer gas has long been practised as a method of heating low vapor pressure metals to achieve high vapor pressure while allowing optical observations of the spectroscopy of the metals through a cool window that does not receive metallic condensation. Initial work was with cylindrical buffered heat pipes, but the need for an angular-dependence measurement lead to the introduction of a disc-shaped buffered heat pipe, which had a cylindrical pyrex window to observe visible fluorescence. The use of any window has to be avoided for efficient collection of 13.5 nm light because all materials are strongly absorptive at this wavelength, so the use of a cylindrical buffered heat pipe with an axial aperture in place of the window was introduced in a prior experiment on the capillary discharge excitation of 13.5 nm radiation in Li. The axial aperture was differentially pumped to allow efficient optical transmission outside of the aperture in a beam tube connecting with a spectrometer. In M. A. Klosner et al., Appl. Opt. 39, pp. 3678-3682 (2000), it was suggested that a micro-capillary array of channels would allow collection of more 13.5 nm light, while maintaining the pressure differential. However, the authors did not show how to collect radiation in a large solid angle (defined as greater than one steradian) while containing the metal vapor. Additionally, the authors did not show a method of introducing the discharge current without use of high temperature ceramic-to-metal seals, which are a difficult technology at the required 800 C temperature, especially when compatibility with Li vapor is needed. Zukavishvili et al., in U.S. Pat. No. 6,933,510, discussed supply of lithium to a discharge via a wick, but did not describe an effective method of containing metal vapor in a wide angle range, so as to protect a collector optic, or arranging for its reflux back into the discharge. Accordingly, it is necessary to introduce an effective means for the production of a useful metal vapor density in plasma discharges for 13.5 nm production at the same time as providing for wide-angle collection of 13.5 nm radiation without metal vapor escape toward the collection optic.